tl;dr, this kind of thing would need heavy adjustment to fit into dda.
This works with portals, since you’re actually visiting an entirely different area every time. Keep in mind that if it’s something like an underground dungeon or an isolated building, this does not work.
Regarding layout, maps need to be vertically consistent, since players might want to excavate between floors depending on the situation.
This makes no sense unless you’re navigating a series of areas connected by portals, which is pushing things a bit far.
This is pretty much right out, monsters need to have a fixed population, or appear according to some logic, magical spawning is not ok.
This is right out, items spawned wihin the dungeons need to follow some kind of logic, random distribution is not ok.
I can’t think of a scenario that both justifies the existince of such a dungen in dda and has shopkeepers.
Should not be a mysterious force, it needs a clear rationale.
You’re correct that this is very different from how loot acquisition normally works in dda. It needs a better justification than “offers a new path for acquiring loot”. Just having more options is not necessarally better.
The concepts aren’t unheard-of in dda, but something like this appearing in a stable, repeatable, and useful manner does not.